The electrical conductivity (.sigma.) of most organic materials at room temperature is quite small (.sigma.&lt;10.sup.-10 ohm.sup.-1 cm.sup.-1). Over the last two decades, the synthesis of organic molecules with electrical properties approaching those of metals have been the focus of considerable attention. Because organic polymers generally have elasticity, strength and plasticity, they offer significant advantages over non-polymeric materials in the manufacture of electronic materials. Macromolecular substances can now be tailored to perform as semiconductors or even as true organic metals.
The field of organic metals is dominated by two types of molecular structures: linearly conjugated .pi.-systems and charge-transfer complexes which form stacks of .pi.-systems in the solid state. In the former systems, electrons move rapidly along a partially oxidized or reduced molecular chain. Examples of linear .pi.-conjugated systems are polypyroles, polythiophenes, polyanilines, polyacetylenes and polyarylenes. In charge-transfer complexes, electrons move along a partially oxidized or reduced stack of molecules. In either case, the electrical, optical and magnetic properties are a complex function of the solid state structure, and efforts have been made to prepare and study model compounds for these systems, primarily in solution.
Conductive polymers which comprise conducting stacks of organic ion radicals or charge-transfer complexes are of considerable interest. The construction of macromolecular complexes from cationic polymers and monomeric anion radicals is by far the most widely utilized route to this type of conductive polymer. The polymeric cations can have a quaternary amino function within the polymer which enables the complexation of anionic acceptor molecules.
Tetracyanoquinodimethane (TCNQ) is a powerful .pi.-molecular acceptor, and its formula is given below: ##STR2## The TCNQ radical ion can be formed by one electron addition to TCNQ and forms organic semiconductors with a variety of cations. In some cases, charge transfer salts are formed in which there is a partial charge transfer between a donor molecule and TCNQ. In other cases, stacks are formed which contain TCNQ.sup.0 and TCNQ.sup.-, along with a polycation. Some of the other acceptors used instead of TCNQ.sup.0 in conjunction with polycations are tetracyanoethylene (TCNE) dichlorodicyanoquinone (DDQ) and chloranil (tetrachloroquinone). The formula of DDQ is shown below: ##STR3## A macromolecular semiconducting complex may be formed by the addition of a solution of a cationic polymer to a solution of a metal salt of the anion radical, from which the polymeric complex often precipitates.
Polyvinylpyridines (PVP) have a conductivity of the order of 10.sup.-15 .OMEGA..sup.-1 cm.sup.-1 at 300K, and this figure is typical of most other nitrogenous polymers. Quaternization of the nitrogenous moiety to yield a polycation, and doping these with acceptor molecules by the coprecipitation method can lead to an increase in the conductivity of the macromolecule by several orders of magnitude, i.e., from 10.sup.-15 up to 10.sup.-3 .OMEGA..sup.-1 cm.sup.-1 by doping PVP with I.sub.2.
In the early 1960s, it was realized that materials of good conductivity could be prepared by mixing a solution of a polycation-halide complex (P.sup.+ X.sup.-) with a solution of Li TCNQ. A careful selection of solvents enabled the immediate precipitation of the exchanged species, P.sup.+.TCNQ . Close inspection of these polymeric salts revealed that the polycation-TCNQ polymers were actually poorly conducting, and that the presence of TCNQ.sup. was central to the presence of conductivity in these polymeric salts.
The conductivity of the P TCNQ complexes (simple salts) can now be adjusted by the introduction of TCNQ.sup.0 into simple salts to form mixed-valence species (complex salts) yielding values as high as 10.sup.-3 .OMEGA..sup.-1 cm.sup.-1, (or 10.sup.-3 S cm.sup.-1), e.g., as reported by J. H. Lupinski et al., J. Polymer Sci, Part C, 561 (1967). Complex salts are prepared by dissolving the simple salt and TCNQ in dimethylformamide and then evaporating the dimethylformamide under reduced pressure. The molar ratios of both TCNQ to P.sup.+, and TCNQ.sup.0 to TCNQ determine the electrical characteristics of the resultant polymer For example, T. Kamiya et al., J. Polym. Sci, Polym. Lett. Ed., 19, 331 (1981) have shown that the conductivities of typical ionene-type simple salts of (5), (6) and (7), of 7.14.times.10.sup.-9, 2.3.times.10.sup.-8, 7.7.times.10.sup.-6 were increased to 2.1.times.10.sup.-3, 2.2.times.10.sup.- 2 and 4.0.times.10.sup.-2 .OMEGA..sup.-1 cm.sup.-1, ##STR4## respectively, by constructing complex salts (TCNQ.sup.0 /TCNQ , 1:1) at the same TCNQ loading level as the simple salts. The activation energies for conduction fell from 0.33, 0.30 and 0.25 to 0.069, 0.089 and 0.074 eV, respectively.
Other authors have performed similar comparisons which agree with these observations. For example, R. Oshima et al., J. Polymer Sci., Part A, 25, 2343 (1987) prepared the conductive TCNQ anion radical salt of poly(1-histinidium) with a 1:1 stoichiometry.
From known crystal structures and conductivity anisotropy of monomeric salts, it is clear that conductivity arises from the motion of odd .pi.-electrons among TCNQ sites. The TCNQ molecules are arranged in face-to-face stacks such that .pi.-orbital overlap within a stack is considerably larger than that between any other near neighbors. Conductivity states arise from the introduction of defects into an ordered ground state configuration induced by an applied potential. The propagation of the electrons is greatest in the direction along the stacks, or normal to the planes of the quinone rings. For simple salts, this process involves an increase in the electrostatic energy, C, but for complex TCNQ salts when both TCNQ and TCNQ.sup.0 are present, C and consequently activation energy E.sub.a are much lower and the conductive properties of these salts is several orders of magnitude greater than for simple salts.
A potentially valuable feature distinguishing these TCNQ-polymer salts from other conducting polymers is their solubility in some organic solvents and their ability to form self-supporting films from solution. Despite conductivities as high as 10.sup.-2 .OMEGA..sup.-1 cm.sup.-1, conductive films composed of TCNQ and cationic polymers are both isotropic and unstable to moisture, thus severely limiting their practical utility.
The existence of conductive polymers which exhibit anisotropy of the conductivity has only occasionally been reported. For example, M. Watanabe et al. Polym. J., 14, 189 (1982) reported that the TCNQ salts of elastomeric ionenes which contain 4,4'-bipyridilium or 1,2-bis(4-pyridinium) rings exhibited a resistivity of ca.10.sup.1 .OMEGA.. These salts could be obtained as flexible films which, when drawn, showed anisotropic conductivity. J. A. Siddiqui et al., Polymer Comm., 28, 90 (1987) reported anisotropic conductivity in oriented films of poly(ethylene oxide)-Na.sup.+ ion complexes with TCNQ . J. Ulanski et al., Synthetic Metals, 35, 221 (1990) reported anisotropic conductivity in films prepared by in situ crystallization of conducting tetrathiotetracene(TCNQ).sub.2 complexes, in a polyethylene matrix during the film casting procedure. The films are prepared by casting the polyethylene solution containing, i.e., 1% of the dissolved charge transfer complex with zone evaporation of the solvent.
Despite the anisotropic conductivity reported for these materials, a continuing need exists for conductive, film-forming complexes between .pi.-donors that readily stack in solution so as to exhibit a high degree of .pi.-orbital overlap, and cationic polymers which enhance the ordered array of the stacked .pi.-donors to yield anisotropic conductivity. A further need exists for convenient methods to prepare such materials. Such complexes can provide films which can act as self-insulated wires, providing the films are sufficiently resilient.